1. Field of the Invention
The present invention relates to an improved chemical reaction apparatus which is capable of removing large heat fluxes from a viscous reaction mixture while maintaining the reaction mixture at essentially isothermal conditions. The invention also relates to a method of conducting chemical reactions, and in particular, polymerization reactions, at essentially isothermal conditions using the novel reactor of the invention.
2. Description of Related Art
A variety of commercially important chemical reactions, and in particular polymerization reactions, require that reactants be maintained within a narrow temperature range to achieve desired product properties.
In the case of certain polymerization reactions, due to the low conductivity and high viscosity of the reaction mixture, heat transfer is a limiting factor in reactor design. Further, it is frequently not possible to compensate for the low conductivity of a polymer reaction mixture by using lower coolant temperatures because low coolant temperatures cause polymer solidification in the reactor. In many polymer reactors, the poor heat transfer characteristics of polymer reaction mixture results in poor reaction temperature control resulting in the formation of undesirable products in the reactor. For example, temperature variations in polymer reactors can lead to the formation of polymer products having lower molecular weight than desired. This negatively affects the flow and mechanical properties of the desired polymer end product. Since it is frequently the case that undesirable polymer reaction products are not easily separated from desired polymer products, many attempts have been made to produce polymer reactors which are capable of controlling, within a narrow temperature range, the reaction temperature of highly viscous reactants with poor thermal conductivity.
A wide variety of designs have been developed for continuous flow polymerization reactors which can handle viscous process liquids with poor thermal conductivity.
U.S. Pat. No. 2,727,884 to McDonald describes a polymer reactor which uses forced convection heat transfer. In this reactor, banks of cooling tubes in which a heat transfer fluid is circulated are gently agitated in the polymer reaction mixture. The agitation improves heat transfer, and at the same time preventing channeling of less viscous material in the reactor. Examples of another type of mechanically agitated, convection-type reactor, known as a wiped-film reactor, are disclosed in U.S. Pat. No. 3,513,145 to Crawford, U.S. Pat. No. 3,679,651 to Kii and U.S. Pat. No. 4,011,284 to Gawne. The construction of the internal coils required by such reactors is extremely labor intensive and therefore very costly. Further, the internal heat transfer coils employed in such reactors have a history of failure causing polymer to be contaminated with heat transfer oil. Despite the agitation in these reactors, the heat transfer characteristic of wiped-film reactors is nevertheless poor and hot spots frequently develop in such reactors.
Another example of a polymer reactor designed to provide high heat removal capability is disclosed in U.S. Pat. No. 3,838,139 to Latinen. This patent describes a horizontal cylindrical reactor vessel equipped with an agitator consisting of a plurality of discs with small clearance with respect to the cylindrical vessel. The discs divide the reactor vessel into compartments. The heat of reaction is removed from the reactor by the direct evaporation of a volatile monomer from the reaction mixture. Although this form of heat transfer is generally accepted as more efficient than convection, hot spots with temperature difference as high as 5–10° C. are still experienced. Further, the temperature in the various compartments of the reactor are not necessarily the same due to different polymer concentrations and reaction rates in the compartments.
Using the vaporization of volatile monomer reactants to remove the heat reaction from a polymer reaction mixture is generally not workable when the polymer reaction involves the co-polymerization of more than one monomer. In such cases, the vaporization of the different monomers is generally not equal causing uncontrolled concentration of the different co-monomers. Consequentially, the use of direct evaporation is to be avoided in such cases.
U.S. Pat. No. 4,419,488 to Fukumoto discloses another direct evaporation-type polymerization reactor. The inclusion of a mechanical agitator in a polymer reactor is often an unwanted necessity to improve heat transfer and homogeneity. These devices are costly, require much maintenance and can cause quality problems because the agitator shaft seal is often a source of air ingress to a reactor, which can generate undesirable oxidation by-products such as aldehydes and ketones. These compounds can retard a polymerization reaction and can cause product discoloration.
Several designs have eliminated the use of mechanical agitators in polymer reactors. U.S. Pat. No. 4,421,162 to Tollar and European Patent No. 0150225 A1 describe the use of flat annular plates disposed coaxially within a reactor shell. This concept can be applied to a polymerization reactor with a viscous reaction mixture by causing the heat of reaction to first be absorbed by conduction through the flat annular plates and then by conductive tubes which are in contact with the annular plates and then by convection into an appropriate heat transfer liquid flowing through the conductive tubes. The temperature in such reactor is usually well controlled, however, the volume occupied by the annular plates and tubes in the reactor reduce available reactor volume significantly. Variations of this heat transfer mechanism has been proposed by other inventors such as Oldershaw in U.S. Pat. No. 3,014,702, Brassie in U.S. Pat. No. 3,280,899, Aneja in U.S. Pat. No. 4,808,262 and Mattiussi in U.S. Pat. No. 5,084,134.
Anionic polymerization reactions have also been conducted in continuous stirred tank reactors. However, such reactions must generally proceed at low temperature due to the extreme reactivity of the reactants. Because it is desirable to operate continuous flow reactors hydraulically full to enable simple process control, heat removal in these reactors cannot depend on evaporation. Consequently, such reactors most often rely upon cooling jackets for heat transfer. However, the effectiveness of cooling jackets on anionic polymerization reactors is constrained by the low heat transfer coefficients applying of the convection mechanism and by the limited range of coolant temperature imposed by polymer solidification temperatures.
It would be desirable if there were available a continuous reactor for viscous polymer reaction mixtures with improved heat removal capability and with the capability of maintaining an essentially isothermal temperature profile throughout the reactor regardless of any varying heat loads associated with different reaction rates or reaction products. It would also be desirable if such a reactor were to be easy to construct, operate and maintain. It would further be desirable if the reactor were to have a relatively large void fraction for the conduct of polymerization reactions in as small a vessel as possible.
These benefits and other advantages are achieved with the present invention.